This invention relates to an improved motor driven roller in which the motor is located inside the roller and, more particularly, to such a motor driven roller in which the motor directly drives the roller.
Motor-driven rollers are used in a variety of applications. Among these applications are the rollers used in exercise treadmills and in material handling conveyors. The specific embodiment of the invention, described in this patent specification, is directed to a conveyor application. However, it will be appreciated the invention is applicable to motor driven rollers used in other systems, such as treadmills.
In a widely used prior art motor driven conveyor roller, a brushless, permanent magnet, d.c. motor is housed inside the roller itself. The motor, which is necessarily limited in size, has a relatively low torque. Typically, a conveyor roller has an inside diameter of less than two inches. A reducing gear is needed to couple the motor rotor to the roller in order to generate the torque required for the conveyor roller application. A resilient clutch is used to uncouple the motor from the roller in those situations where the roller becomes stuck. While generally satisfactory, the reducing gear requires maintenance and is subject to breaking down, which requires disassembly of the roller and repair or replacement of the broken parts.
An object of this invention is the provision of an electronically controlled, high torque d.c. motor assembly housed inside the roller and directly connected to it, which eliminates the need for a reduction gear and a clutch control used in the prior art.
Briefly, this invention contemplates the provision of a motorized roller in which a cylindrical permanent magnet is secured to the inside surface of the roller. Longitudinal segments are magnetized to form poles of alternate north and south magnetic polarity. These magnet poles are the rotor of an inside-out brushless d.c. motor, the stator of which is formed by coils in slots in a toothed structure mounted on a stationary shaft about which the permanent magnet rotor and the roller to which it is attached rotates. Preferably, the number of rotor poles is, or is close to, the maximum number of poles that can be formed about the circumference of the cylindrical permanent magnet, given the constraint on the diameter of the permanent magnet since it much fit within the roller, and the constraint of practical manufacturing limitations. Increasing the number of magnetic poles decreases the required thickness of the back iron which is needed to generate a high flux density in the air gap, which in turn is necessary to generate a high torque output per unit volume. It will be appreciated that the required back iron thickness is approximately equal to the ratio of the number of magnetic flux lines per pole to the acceptable back iron flux density level. As the number of poles increases, the magnetic lines per pole decrease, since the magnetic flux is evenly distributed among the poles.
The stator coils are electronically commutated to provide brushless operation. One end of the stator shaft extends beyond the end of the roller and is secured to a suitable frame member. Wires in a passage in the shaft carry current to the coils. Preferably, six-step switching is used to commutate the stator coils and the commutation angle can be advanced as the motor speed increases in order to maintain a desired torque. In one embodiment, the motor extends the length of the roller. In another embodiment, the motor extends for only a part of the length of the roller. The permanent magnet in which the poles are formed may be secured to the inside of the roller by means of a suitable adhesive. Here, the roller itself serves as the back iron to provide a low reluctance path to complete the magnetic circuit between adjacent poles. In another embodiment, the entire motor assembly is secured in a metal housing, which is then secured to the roller by force fit or other suitable means. Here the metal housing serves as the back iron member.